This invention relates generally to a process for producing microstructured composite articles, and more particularly to a process for producing such articles by applying resin composition used in the article in lines on a microstructured molding surface. The resin composition wicks and flows along grooves in the microstructured molding surface to coat the surface before the resin composition is cured, thereby resulting in a composite article that is free of air bubbles, and that is able to be made rapidly. The molding surface or the resin composition, or both, may be optionally heated to allow the resin composition to more quickly and easily fill the grooves in the molding surface.
Microreplication is a process used to make and duplicate microstructured surfaces. Microreplication is used to produce a wide range of products, one example being optical films. Microreplication of optical films, for example, can dramatically alter the optical properties of the surface.
In general, it is difficult to replicate high-quality, microstructured surfaces at rapid line speeds. The problem that occurs at rapid line speeds is that air bubbles are trapped in the replicated microstructure, thereby affecting the properties of the surface of the end product. The air bubbles are a result of air being trapped in recesses in the negative molding surface used for replication while the molding surface is being coated with the resin composition that forms the microstructured surface. Effectively, the resin composition does not displace all the air in the recesses and air bubbles result.
A well-known coating method used to replicate a microstructured surface is a continuous coating method. U.S. Pat. No. 3,689,346 (Rowland) teaches a process for continuous replication of retroreflective cube-corner sheeting by applying a layer of crosslinkable, partially polymerized resin over a negative molding surface to be replicated, and exposing the resin to actinic light or heat to solidify the resin. This process does result in replicated surfaces without air bubbles at very slow line speeds. However, air bubbles result from the process at rapid line speeds.
There is another method that is used to replicate microstructured composite articles. This process, as described in Japanese Patent Laid-Open Publication No. 9301/1991 describes a process for producing a lens sheet, comprising the steps of coating a first radiation curable resin onto the entire surface of a forming die in the shape of a flat plate; forming a resin bank of a second radiation curable resin on the first radiation curable resin; and superposing a laminating base material on the second radiation curable resin; and uniformly laying the second radiation curable resin over the first radiation curable resin while forcing out, by the use of a pressure roll, those air bubbles that enter between the base material and the forming die. Although this method may eliminate the air bubbles, it is so time consuming that it does not allow for production of articles at a high rate of speed.
The inventors recognized the need for a process that allows high-quality microstructured composite articles, that are free of air bubbles, to be produced at rapid line speeds. The present invention meets the need that exists for such a process.
A first aspect of the present invention is a process for producing a microstructured composite article comprising: providing a tool with a molding surface that is suitable for forming a microstructure; applying to at least a portion of the molding surface a plurality of continuous lines of a flowable, curable resin composition; contacting the resin composition with a substrate; curing the resin composition; and, transferring the resin composition to the substrate.
A second aspect of the present invention is a process for producing a microstructured composite article comprising: providing a tool comprising a molding surface comprising a plurality of grooves that are suitable for forming a microstructure and that are non-circular in shape; applying to at least a portion of said molding surface a plurality of lines of a flowable, curable resin composition in a direction that is non-parallel to said plurality of grooves; contacting said resin composition with a substrate; curing said resin composition; and transferring said resin composition to said substrate.
A third aspect of the present invention is a process for producing a microstructured composite article comprising: providing a tool comprising a molding surface that is suitable for forming a microstructure; heating said molding surface; applying to at least a portion of said molding surface a plurality of lines of a flowable, curable resin composition; contacting said resin composition with a substrate; curing said resin composition; and transferring said resin composition to said substrate.
Another aspect of the present invention is the process as described directly above in the third aspect, and further comprising heating said flowable, curable resin composition before applying said resin composition to said molding surface.
A further aspect of the present invention is a process for producing a microstructured composite article comprising: providing a tool comprising a molding surface comprising a plurality of grooves that are suitable for forming a microstructure; applying to at least a portion of said molding surface a plurality of lines of a flowable, curable resin composition; applying to at least a portion of a substrate said flowable, curable resin composition; contacting said resin composition on said molding surface with said substrate after said resin composition is applied to said substrate; curing said resin composition; and transferring said resin composition on said molding surface to said substrate.
One advantage of at least one embodiment of the present inventive method is that it is successful in producing microstructured composite articles that are free of air bubbles. It also allows them to be made at rapid line speeds.
Another advantage of the present inventive method is that the coating die used to deposit the resin composition on the molding surface does not need to be positioned as close to the molding surface as in other processes. In other processes, coating a continuous layer of resin onto the tool requires the die to be close to the molding surface, which increases the potential of damaging the tool.
The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. The figures and the detailed description, which follow, more particularly exemplify those embodiments. In addition, the above and other advantages are more fully shown and described in the drawings and detailed description of this invention. It is to be understood, however, that the drawings and description are for purposes of illustration only and should not be read in a manner that would unduly limit the scope of this invention.
In this application:
xe2x80x9cbubble-freexe2x80x9d or xe2x80x9cfree from air bubblesxe2x80x9d or xe2x80x9cfree of bubblesxe2x80x9d refer to the microstructured composite article produced by the present inventive process being free of, or without any, air bubbles as observed by the unaided human eye under ambient light conditions:
xe2x80x9ccross-hatchedxe2x80x9d means two sets of grooves that are intersecting and cross each other;
xe2x80x9ccross webxe2x80x9d refers to a direction about perpendicular to the down web direction;
xe2x80x9cdown webxe2x80x9d refers to the lengthwise direction of the web, or the direction of the web that extends in the machine direction, which is the direction of movement of the substrate as it is moved in the present inventive method;
xe2x80x9cflowablexe2x80x9d means capable of moving freely in the manner characteristic of a fluid;
xe2x80x9cgroovexe2x80x9d means a long and narrow furrow, channel, depression, hollow or indentation; and
xe2x80x9cmolding surfacexe2x80x9d means the surface of an object used to give a particular shape to a substance.